Not sure what's happened, but nothing's been writing to the database since January. I've restored a backup from December and existing accounts now work as they should and new accounts are able to be created again.

I think this is something I need to look into further as some accounts have no statistics saved to them.

Edit:
Scratch that, I have plans to manually add new property + vehicle locations so existing ones have been scrapped, thus needing player accounts to also be reset. If you still play, hit me up, I'll still be able to individually restore your account.